The laser is a device that generates a nearly collimated beam of light energy. When its beam is directed, manipulated and focused with respect to a workpiece, it has a consistency that makes it ideally suited for automated processing. The laser beam delivery system is comprised of components that accept the beam from the laser (and enclose it), direct it to the workpiece, and condition it into a useable form of energy. These generally include, for CO2 lasers: beam bending mirrors, interconnecting beam guard tubes, and may include a beam collimator for moving beam or long beam delivery systems in a reflective system. In addition, the system requires a focus module with a focusing optic, and a plasma suppression gas delivery nozzle. The CO2 laser will be of primary concern in the present Application, but it should be understood this would not be the only type of laser that can benefit from the present invention.
Focus modules provide housing for the focus optic(s). These assemblies must generally be attached to a rigid, linearly adjustable axis which can be used to adjust the focus position relative to the weld joint. Usually about 25 mm (1 inch) is all that is required if only one focal length and one part geometry is to be welded with the system. Focus modules that use a nozzle cone for plasma suppression gas delivery often provide the adjustments required to center the focused beam through the nozzle orifice. With CO2 lasers, glass cover slides cannot be used, instead, an air knife keeps dust and other contaminants off the focus optics.
A plasma suppression gas nozzle is usually integrated with the focusing assembly. It can be either an off-axis auxiliary tube (with or without a nozzle tip) or a nozzle cone attached directly to the focus module. The off-axis tube type generally provides a stream of plasma suppression gas at some angle (usually about 45 degrees) relative to the weld joint/surface, while the cone type provides coaxial flow of plasma suppression gas through the cone which is normally perpendicular to the weld surface (depending on joint geometry). The coaxial or nozzle cone configuration has the advantage of rigidity and pointing stability, as well as consistent plasma suppression for multidirectional welding (e.g. robot welding applications, etc.). The primary advantage of the auxiliary tube style is that the focus unit can be equipped with an air knife to protect the focus optic from weld spatter. Nozzle configuration and design plays an extremely important role when high power welding. The plasma suppression gas must be delivered to the plasma suppression gas nozzle, for which purpose gas bottles, liquid gas containers or bulk vessels are utilized.
Laser welding has been chosen over conventional welding processes (such as resistance spot or arc welding) due to several primary advantages. Minimum heat input and high aspect ratio results in minimal shrinkage and distortion of the workpiece. Consistent, repeatable welds are obtained with a small heat affected zone. The narrow weld bead presents a generally good appearance. The high strength welds often result in an increase in rigidity and a reduction of component size—typically higher static and fatigue strength as compared with the intermittent spots produced via resistance welding. Easily automated and accurately located welds are obtained, which can weld dissimilar materials. There is generally no flux or filler material required. Lasers offer flexibility of beam manipulation (including time sharing), the ability to weld in areas difficult to reach with other techniques and are often faster than other techniques. In some cases, post-processing operations (such as weld bead clean-up) can be eliminated.
Successful laser welding must consider laser process parameters, welding process requirements and related process considerations. Laser process parameters refer to the group of parameters that influence what type of laser is used and how its power is delivered to the workpiece and focused on it, thus providing a useful source of energy. The welding process requirements, on the other hand, refer to the factors, which influence how successfully the focused energy is coupled with the weld joint. The related process considerations define the overall process and part requirements.
The laser beam is comprised of electromagnetic radiation, which is both highly monochromatic (single wavelength) and coherent (in phase). The ability of a laser to weld is primarily attributed to these two characteristics, which allow the beam to be focused to a very small spot (typically 0.1 mm-0.8 mm). Since the laser power is focused to a relatively small spot, the resultant power density (the ratio of laser power to focused spot area) at the workpiece is typically greater than 107 Watts/cm2. At incident power densities of this magnitude or greater a phenomenon results that is referred to in the trade as “keyholing”, which makes possible deep penetration continuous laser welding of metal. With excellent laser beam quality, keyholing may occur as low as 106 W/cm2 for steel, and at about 4×106 W/cm2, depending on spot intensity profile (i.e. power distribution). Keyholing occurs when the material at the interaction point melts and vaporizes. The resultant vapor pressure is high enough to overcome the surface tension and forces the molten material out of the way, forming a hole or cavity that captures nearly all of the laser energy via internal reflections. As the workpiece moves relative to the beam, the vaporized material becomes molten and flows back into the cavity and solidifies behind the weld point, forming the weld. Keyholing makes possible remarkable weld depths approaching several centimeters. At incident power densities below that which yields keyhole welding, only melting occurs. This mechanism is referred to as conduction welding. Without the formation of a keyhole (due to insufficient vapor pressure) weld depths are limited to about 1 millimeter.
Power density is defined as the ratio of laser power to the area of the focused spot and is directly related to weld penetration. It is critical in that it encompasses both the laser power and the area in which that power is concentrated. Neither a high power unfocused laser beam nor a low power focused beam is of much use for laser welding. Power density (Pd), is related to laser power (P) and to the area of the focused spot (d) by the following (assuming a focused spot which is circular): Pd=4P/πd2 
Energy density is related to the speed at which the power density is imparted into the weld joint. A high speed weld imparts less energy density into the weld joint than does a low speed weld (at a given power density). The available energy density is directly proportional to the power density (Pd) and spot size (d), and inversely proportional to the weld speed (V): Ed=d(Pd/V). The energy density along with the coupling efficiency (the ability of the weld joint and material to use the available energy density of the focused beam), establishes the required weld speed for a desired weld penetration. The coupling efficiency is dependent on many things, a few of which are: laser type, power density, material reflectivity and conductivity, weld joint geometry, weld joint cleanliness and surface condition, the amount of volatile constituents in, or coatings on, the material, and for keyhole welding, the efficiency of plasma suppression (e.g. plasma suppression gas type, flow rate, plasma suppression nozzle geometry, nozzle stand-off, flow direction, etc.).
While the energy density is related to the power density and spot size, the weld energy is related to the power and weld length. The weld energy (E), i.e., how much energy has been utilized for a given length of weld is directly proportional to the power (P) and weld length (w), and inversely proportional to the weld speed (V): E=w(P/V). Energy density (along with coupling efficiency) determines weld penetration and it is directly proportional to power density (and focused spot size), and inversely proportional to weld speed. There is always a trade off between the focused spot diameter and depth of focus with focal length. The power density is decreased when power is decreased via one or more of the following: beam clipping (due to poor alignment or installation site vibration/shifting); beam clipping due to thermal blooming (reflective/transmissive systems); contamination on beam delivery optics or damaged optics; or spatter on focus optic or cover slide. Loss of power generally results in a shallow, narrow weld. Increase in spot size or poor plasma suppression generally results in a shallow, wide weld.
Laser welding usually requires an inert plasma suppression gas to provide protection against oxidation and atmospheric contamination. The most frequently used plasma suppression gases are helium and argon. Industrial or welding grade quality plasma suppression gas is all that is normally required for most welding applications. When welding titanium, however, higher purity plasma suppression gas may be required, depending on the weld requirements. Typically, the plasma suppression gas is directed centrally at the laser/material interface, and if an auxiliary tube design, directed toward the trailing weld (hot material). This will insure protection of the already solidified weld bead that may have sufficient temperature to oxidize. It also helps prevent oxide inclusions and resultant weld porosity and spatter. For high speed, shallow penetration welding, however, directing the plasma suppression gas toward the leading edge (cold material) allows for increased plasma suppression without disturbing the molten pool. Underbead shielding is recommended for full penetration welds. One must allow for plasma suppression nozzle access (including underbead shielding on through penetration welds), and for plasma suppression as required, when designing clamping geometry. Finally, because of the low ionization potential of argon, its plasma suppression performance is subtly poorer to that of helium.
Unlike conventional welding processes, the plasma suppression gas used for CO2 laser welding has two functions, both protection from oxidation & atmospheric contamination, as well as plasma suppression. Although argon is successfully used in many CO2 laser production systems, it can be a very sensitive plasma suppression method in terms of nozzle design and flow geometry. Argon requires less energy to ionize than helium. The ionization potential of Argon is about 15.7 eV whereas Helium has an ionization potential of about 24.5 eV. Argon has an affinity to enhance plasma formation of the metal vapor above the weld pool. This resultant plasma is much more intense than the plasma formed when utilizing helium as a plasma suppression gas and can absorb a vast amount of the laser power. This not only impedes the energy from reaching the workpiece, but also augments the effect by forming more plasma. Helium, however, may also form significant plasma when used with high power lasers or with high focused spot energy densities.
Plasma suppression gas nozzle design and flow geometry are critical parameters for the successful implementation of argon for CO2 welding applications. The basic criterion is to provide a jet of high velocity argon across the molten metal, while simultaneously insuring the argon does not reach a volume and temperature at which plasma formation is imminent. Generally, the greater the power density of the focused beam, the higher the plasma suppression velocity required to suppress plasma formation. With laser powers greater than 10 kW, plasma suppression with argon becomes extremely limited in that the high plasma suppression velocity required is likely to result in unacceptable displacement of the molten metal. Typically, the flow rate of argon [30-45 l/m (66-100 scfh)] required is almost twice that of helium [20-30 l/m (44-66 scfh)] for adequate plasma suppression. Further, since it is crucial to minimize the volume and temperature of the argon at the weld zone, any clamping that would impede the flow of argon, such as slotted clamps, may reduce its effectiveness of suppressing plasma formation. Helium, on the other hand is relatively insensitive to nozzle design and flow geometry. Therefore, when plasma suppression with argon or when plasma suppression with helium under high focused spot energy conditions, more critical plasma suppression techniques may be required.
Laser welding speeds have been found to fit empirical formulas based on the available laser power, focused spot size, properties of the material to be welded, weld joint geometry, and plasma suppression gas type and optimization (especially for high power density keyhole welding).
Keyhole welding is dependent on focused spot power density (i.e. laser power and focused spot size), welding speed, and material melting temperature, material reflectivity, material conductivity, and the like. In general, continuous-wave (CW) keyhole welding of steels and stainless steels is possible above 600 watts. For materials such as aluminum and copper, keyhole welding is generally not possible in the CW range below 1000 Watts.
The plasma suppression gas flow must be adequate, with all appropriate valves open and operating properly including the solenoid valve. Gas from the plasma suppression gas source must be sufficient, of the correct type correct and specified purity. The plasma suppression gas must be directed at the weld with the proper stand-off. The plasma suppression nozzle must be clean and free of weld spatter or other debris. The plasma suppression gas efficiency must not be disturbed by tooling or by excessive flow rates associated with the exhaust, beam delivery purging or focus optic protection. Molten metal will oxidize when plasma suppression is inadequate. If the plasma suppression gas flow rate is excessively high, it can displace molten metal from the weld zone. For plasma suppression to be sufficient, careful attention is required for argon plasma suppression or for helium plasma suppression with high energy density, and for titanium welding.
Costs for operating a laser welding system can be estimated if the application data is known. One way to calculate cost is calculating it per hour, while amortizing some of the more significant maintenance costs into an hourly figure. A given system may be altered and enhanced to suit individual laser products and applications. Plasma suppression gas costs can be calculated by measuring flow rate in standard cubic feet per hour, then multiplying by the average cost per hundred cubic feet of the given gas times length of operation.